1. Technical Field
The present invention relates to an organic electroluminescence device, a method for the driving thereof, and an electronic appliance.
2. Related Art
In recent years, organic electroluminescence (hereafter “organic EL”) devices that include organic EL elements have been featured as a light-emitting element that does not require a light source such as backlight. An organic EL element has an organic EL layer, or in other words, a light-emitting element between a pair of counter electrodes that face each other. Organic EL devices for a full-color display have light-emitting elements that have a range of light-emitting wavelengths, each corresponding to red (R), green (G), and blue (B). When a voltage is applied between a pair of counter electrodes, implanted electrons and implanted holes recombine within a light-emitting element, thereby emitting light. The light-emitting element formed in these organic EL devices is normally formed in a thin film less than 1 μm thick. The backlights used in common liquid crystal display devices are not required in the organic EL devices, since the light-emitting element itself emits light. Therefore, it is an advantage for the organic EL devices, that the thickness thereof may be significantly thinner.
The luminescence property of the aforementioned light-emitting element is affected by: an external environment, molecules constituting an emissive layer, and a minute amount of impurities present in the emissive layer. For instance, impurity ions are diffused within the emissive layer, and are accumulated in a specific location, if the light-emitting element is driven by a direct current, where unidirectional bias is applied. This involves a problem that the accumulated impurity ions trap the holes or the electrons implanted from the electrode, thereby decreasing the luminescent life and the brightness of the light-emitting element. Moreover, the electric field, generated by a forward bias applied during the light emission, may cause orientation polarization or ionic polarization of ionic impurities. The direction of the internal electric field in the emissive layer, in which the orientation polarization and the ionic polarization have occurred, is opposite to the direction of the electric field generated by the forward-biased voltage required for light emission. Hence, the voltage applied during the light emission does not work effectively. This mechanism causes the luminescence property of the light-emitting element to deteriorate.
An example of impurities include ions of indium that constitutes indium tin oxide (hereafter “ITO”). In some heterostructure light-emitting elements, a functional layer adjacent to the emissive layer, and the emissive layer, are doped with impurities by their inner diffusion. Moreover, the impurity ions from the electrode may be doped into the emissive layer, if the electrode (cathode) composed with lithium fluoride (LiF) or calcium (Ca) is adjacent to it. If there is an insufficient refinement of organic compounds used for the emissive layer or the functional layer, the material used upon synthesis may reside as impurities. Moreover, there is a problem that the accumulated charges within the light-emitting element decrease the luminescent life and the brightness of the light-emitting element, when performing a direct current drive where the unidirectional bias is applied.
Considering the above problems, as disclosed in JP-A-9-293588 and JP-A-2004-114506 for instance, there are reports in which an alternating-current drive is used. Here, the driving voltages are forward-biased and reverse-biased, where the reverse-biased voltage has a reverse polarity of the forward-biased voltage, both of which are alternately applied to the light-emitting element during the light emission. Since voltages with different polarities are alternately applied to the emissive layer, this alternating-current drive moderates the accumulation of charges and impurity ions inside the light-emitting element, and the internal electric field generated by the impurity ions. Therefore, the decrease in luminescent life and brightness is suppressed.
However, the light-emitting element in the aforementioned alternating current drive normally has a multilayer structure including an anode, an emissive layer, and a cathode. Hence, the light is emitted only when the forward-biased voltage is applied, i.e. when a positive voltage is applied from the anode and a negative voltage is applied from the cathode. That is to say, the light-emitting element does not emit light when the reverse-biased voltage is applied using the alternating current. As described, this has been a cause of another problem that the shorter the effective light emission time is, the darker the display becomes.